The present invention relates generally to phase noise measurement systems, and more particularly, to a phase noise measurement system that uses three independent signal sources to statistically derive the power spectral density of the phase noise content of each of them.
Typical phase noise measurement test sets available in the commercial market fall into two primary categories: a two oscillator systems (or phase detector system) and a delay line discriminator system. To clarify these systems, each is discussed separately. The two oscillator system uses two sources (a unit under test and a reference source) that are set to the same frequency and that are in quadrature with respect to each other. These signals are input to a double balanced mixer that is used as a phase detector, and the resultant output is passed through a low pass filter. The remaining signal is a low voltage DC signal that is summed with an AC signal. The AC fluctuations are proportional to the combined phase noise of the original two signal sources. This AC signal is then fed into a spectrum analyzer and the power spectrum is displayed to a user. The primary limiting factor of this approach is the requirement that the reference source have phase noise characteristics that are at least 10 dB better than the source that is measured. In the case of phase noise measurements for radar signal sources, these sources have very low phase noise, and finding a reference that is superior can be very difficult or impossible. Therefore, this method is primarily used for measuring signal sources of higher noise content or for measuring phase noise close to the carrier.
The delay line discriminator system does not require additional reference sources. It uses the signal from the unit under test and splits it into two signals. The signal in one pats is input into a delay line whose output feeds a phase detector. The other signal is fed directly to the phase detector. Phase detecting the delayed and non-delayed signals together creates a discrimination effect which produces a frequency modulated signal proportional to the signal's inherent noise content. This FM noise signal is then inteegrated and measured by a baseband spectrum analyzer. This system has several limitations. Its sensitivity is proportional to the delay time and the larger the delay time the greater the insertion loss. This acts as a practical limitation for the sensitivity of the system. Furthermore, the sensitivity degrades as 1/f.sup.2 as the carrier is approached. Therefore, this technique is not useful measuring very stable sources close to the carrier frequency.
Accordingly, it is an objective of the present invention to provide for an improved phase noise measurement system that overcomes the limitations of conventional systems.